Polyesters derived from terephthalic acid or its derivatives are known to possess a spectrum of properties which make them suitable for a multiplicity of fiber applications. The polymer in this category which has found the widest commercial application is polyethylene terephthalate (PET) which has in general excellent mechanical and other properties important in the use of fibers and which can be easily melt spun into a variety of useful fiber products. However, despite the general utility of PET in fiber applications, there exist uses for which somewhat different properties are required than are possessed by PET fibers, e.g. a higher than usual degree of resilience or stretchability as required for example, for certain types of stretch fabrics, pile carpet or fiberfill. For these applications, it has been found that polytetramethylene terephthalate, sometimes referred to as polybutylene terephthalate (PBT) may better serve the purpose than PET.
Connected to these differences in properties between PET and PBT is an inherent characteristic of fiber forming PBT polymers not possessed by PET. Thus, melt spun fibers of PBT have been found to have two crystal forms which have been designated as "alpha" and "beta." Moreover, PBT fiber undergoes a reversible stress induced crystal transition between these forms, the beta form being obtained when a sufficiently high stress is applied externally to the fiber and returning to the alpha form upon removal of the load. This crystal transition occurs fairly sharply at a strain of about 4 to 12% resulting in a plateau in the stress-strain curve of the fiber in this area. It has been found that the beta crystal form prevalent when the externally applied stress exceeds the latter strain range is more elongated than the alpha form. Moreover, release of the stress and transition of the PBT to the alpha form causes some reduction in fiber length due to this transition independent of the normal reduction resulting from the elimination of crystal deformation associated with a normal, linear stress-strain curve. The crystal transition under stress occurs more readily in both directions at elevated temperatures near or above the glass transition temperature of the polymer, than at lower temperatures.
The phenomenon of crystal transition in PBT fibers is variously documented in the literature including the following articles: I. H. Hall et al, "Chain Conformation of Poly(tetramethylene terephthalate) and its Change with Strain" published in Polymer, 1976, Vol. 17, 807-815; and K. Tashiro et al, "Solid-State Transition of Poly(butylene terephthalate) Induced by Mechanical Deformation," published in Macromolecules, 1980, Vol. 13, 137-145.
While PBT fibers produced by known methods are often satisfactory in applications requiring a relatively high degree of stretchability and resilience, there are nevertheless limitations to its production and properties under certain circumstances which it would be desirable to overcome and these limitations are related to the property of crystal transition under stress described previously. Thus, there is a limitation to the spinning speed, i.e. initial wind-up speed of PBT using yarn tensions normally utilized for this polymer, since the frictional and air drag on the fiber at high speeds can cause the strain on the solidified but still hot fiber to exceed the critical stress for the alpha to beta transition resulting in the beta crystal form as it is wound on a bobbin. Subsequently, as the fiber cools, it tends to convert to the alpha form with its attendant shrinkage. This causes a substantially increased force on the bobbin which can become crushed if it is not sufficiently rigid. As a result of this phenomenon, the wind-up speeds employed in the melt spinning of PBT often does not exceed about 3000 meters/min. since conventional bobbins would tend to be crushed if higher speeds were employed.
Another limitation connected to the use of PBT is its degree of stretchability in end use products, particularly in crimped or textured form. While such degree of stretchability is considered adequate and even superior for many applications, there exist other applications for which an even greater degree of stretchability would be desirable.